Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs).
Wireless communication systems are distinguished over fixed communication systems, such as the public switched telephone network (PSTN), principally in that mobile stations move among BTS coverage areas, and in doing so encounter varying radio propagation environments.
In a wireless communication system, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area is defined by a particular range where the BTS can maintain acceptable communications with MSs operating within its serving cell. Coverage areas for a plurality of BTSs can be aggregated for an extensive coverage area. An embodiment of the present invention is described with reference to the Third Generation Partnership Project (3 GPP) defining portions of the Universal Mobile Telecommunication Standard (UMTS), including the time division duplex (TD-CDMA) mode of operation. 3GPP standards and technical release relating to the present invention include 3GPP 25.211, TR 25.212, TR 25.213, TR 25.214, TR 25.215, TR 25.808, TR 25.221, TR 25.222, TR 25.223, TR 25.224, TR 25.225, TS 25.309, TR25.804, TS 21.101, and TR 21.905 hereby incorporated within this application, in their entireties by reference. 3GPP documents can be obtained from 3GPP Support Office, 650 Route des Lucioles, Sophia Antipolis, Valbonne, FRANCE, or on the Internet at www.3gpp.org.
In UMTS terminology, a BTS is referred to as a Node B, and subscriber equipment (or mobile stations) are referred to as user equipment (UEs). With the rapid development of services provided to users in the wireless communication arena, UEs can encompass many forms of communication devices, from cellular phones or radios, through personal data accessories (PDAs) and MP-3 players to wireless video units and wireless internet units.
In UMTS terminology, the communication link from the Node B to a UE is referred to as the downlink channel. Conversely, the communication link from a UE to the Node B is referred to as the uplink channel.
In such wireless communication systems, methods for simultaneously using available communication resources exist where such communication resources are shared by a number of users (mobile stations). These methods are sometimes termed multiple access techniques. Typically, some communication resources (say communications channels, time-slots, code sequences, etc) are used for carrying traffic while other channels are used for transferring control information, such as call paging, between the Node Bs and the UEs.
It is worth noting that transport channels exist between the physical layer and the medium access control (MAC) in the system hierarchy. Transport channels can define how data is transferred over the radio interface. Logical channels exist between MAC and the radio link control (RLC)/radio resource control (RRC)layers. Logical channels define what is transported. Physical channels define what is actually sent over the radio interface, i.e. between layer 1 entities in a UE and a Node B.
A number of multiple access techniques exist, whereby a finite communication resource is divided according to attributes such as: (i) frequency division multiple access (FDMA) in which one of a plurality of channels at different frequencies is assigned to a particular mobile station for use during the duration of a call; (ii) time division multiple access (TDMA) whereby each communication resource, say a frequency channel used in the communication system, is shared among users by dividing the resource into a number of distinct time periods (time-slots, frames, etc.); and (iii) code division multiple access (CDMA) whereby communication is performed by using all of the respective frequencies, at all of the time periods, and the resource is shared by allocating each communication a particular code, to differentiate desired signals from undesired signals.
Within such multiple access techniques, different duplex (two-way communication) paths are arranged. Such paths can be arranged in a frequency division duplex (FDD) configuration, whereby a frequency is dedicated for uplink communication and a second frequency is dedicated for downlink communication. Alternatively, the paths can be arranged in a time division duplex (TDD) configuration, whereby a first time period is dedicated for uplink communication and a second time period is dedicated for downlink communication on an alternating basis.
Present day communication systems, both wireless and wire-line, have a requirement to transfer data between communications units. Data, in this context, includes signaling information and traffic such as data, video, and audio communication. Such data transfer needs to be effectively and efficiently provided for, in order to optimize the use of limited communication resources.
Recent focus in 3GPP has been on the introduction and development of an “enhanced uplink” feature to provide fast scheduling and allocation of system resources for uplink packet-based data, and to serve as a compliment to HSDPA (high-speed downlink packet access). Within HSDPA (downlink), a scheduling (or downlink resource allocation) entity is placed in the Node-B network entity (previously scheduling was performed by a Radio network controller, RNC). The scheduler resides within a new MAC entity termed the MAC-hs.
For HSDPA, scheduling is generally distributed among Node-Bs and downlink soft handover (macro-diversity) is not supported. That is to say that a scheduler exists in each cell which is largely, or wholly unaware of scheduling decisions made in other cells. Each scheduler operates independently. Feedback is provided to the scheduler from the UE in the form of Channel Quality Information (CQI). This information enables the scheduler to accommodate each users particular C/(N+I) (i.e. carrier to noise plus interference power ratio) situation. If schedulers in other cells are generating interference to a UE, this is reflected in the CQI report to the UE's serving cell scheduler, and link parameters may be adjusted in response by the scheduler to maintain an acceptable quality or reliability of radio communication between the base station and the UE. Examples of parameters which may be adjusted in accordance to the UE CQI feedback include: (i) the data rate; (ii) the transmit power; (iii) the modulation format (QPSK/16-QAM); and (iv) the degree of FEC coding applied
An enhanced uplink feature was first implemented for the FDD 3GPP variant. In this case, a scheduler is placed in the Node-B (inside a so-called MAC-e function). As a result of the scheduling function being located in the Node-B, scheduling is largely decentralized. However, because uplink signals from a UE may significantly interfere with the operation of other cells, some degree of co-ordination is required between schedulers of different cells.
Soft handover is also supported for uplink in FDD, and this too requires some control or feedback to the UE from all base stations actively receiving its transmissions. This can similarly be thought of as a form of scheduler coordination between cells.
In reference to FIG. 1a, coordination between cell schedulers has been provided for the FDD enhanced uplink by means of non-serving cells (i.e. cells 003 and 004 in the “active set” but which are not the primary controlling cell 002) providing feedback to the UE 001. The “active set” is defined as the set of cells actively receiving the uplink transmission from the UE 101. Due to the fact that in FDD WCDMA, uplink signals from each user interfere with those of other users, the transmission from UE 101 causes some degree of interference in cells 003 and 004. There is no explicit direct, co-ordination between Node-B's of the active set (002, 003, and 004)—the coordination is effected via the control feedback to the UE.
Control of the UE transmission power and data rate takes the form of grant commands sent from multiple cells to the same UE. The UE receives an “absolute” grant from the serving cell, and may also receive “relative” grants from neighboring cells in the active set. The absolute grant channel (E-AGCH) 007 is used by the serving cell scheduler to convey information to the UE about which resources it may use. Uplink resources are generally thought of in FDD WCDMA as “Rise-over-Thermal” (RoT) resources wherein an allowable received-interference level threshold is set for the base station (relative to thermal noise in the receiver) and each user is effectively granted a fraction of this allowable received interference power. As the allowable RoT set-point is increased, so the interference level at the base station increases and the harder it becomes for a UEs signal to be detected. Thus, the consequence of increasing the RoT is that the coverage area of the cell is reduced. The RoT set-point must therefore be configured correctly for a given deployment to ensure the desired system coverage is met.
If a user is located close to a cell boundary, his uplink transmissions may contribute significantly to the received interference levels observed in a neighbor cell and may cause an allowable interference target in that cell to be exceeded. This can reduce the coverage and degrade the radio communication in that neighbor cell. This is an undesirable scenario, since decisions made by one scheduler in one cell may have a detrimental (and sometimes catastrophic) impact on the coverage or throughput in another cell. Some form of preemptive or reactive action is therefore required to accommodate for this scenario.
For the FDD WCDMA enhanced uplink, reactive (rather than preemptive) action is taken. The reactive action takes the form of the E-RGCH feedback commands 005, 006 from the neighbor cells 004 and 003, respectively, which can be used by a particular scheduler to reduce the UE transmit power when the uplink signal is causing excessive interference in that schedulers' cell.
Thus, uplink interference coordination can be achieved between schedulers but without explicit need for direct inter-Node-B communication. This is beneficial since a distributed scheduling architecture may be retained on the network side (where schedulers do not need to communicate with each other), and this enables the schedulers to be located in the Node-B which can facilitate faster scheduling, lower latency and faster response to retransmissions. When Hybrid ARQ is used (H-ARQ) this is also advantageous since retransmissions can be combined in a soft buffer in the Node-B, obviating the need to relay soft information over the Node-B/RNC interface (Iub).
Uplink soft handover between cell sites is typically not supported for TDD. Nor is the UE currently required to decode information sent on a downlink from any cell other than the serving cell. Thus, the FDD solution to control intercell interference levels throughout the system using E-AGCH from serving cells and E-RGCH from neighboring cells is not appropriate for TDD enhanced uplink. A requirement for the UE's to listen to commands from multiple cells could be introduced, enabling the same E-RGCH feedback scheme to be used. However, this would significantly increase the UE receiver complexity and for this reason, this is not an attractive solution. In reference to FIG. 1b, UE 011 is in TDD communication 017 with its serving Node-B 012, however the UE 011 uplink also causes interference to neighboring cells served by Node-B's 013 and 014.
Other mechanisms for controlling uplink intercell interference must therefore be sought. It is again advantageous to find solutions to this problem that can operate within a distributed scheduling architecture in which a scheduler exists for each cell, or for each Node-B, that may operate independently of schedulers for other cells. This is so that the benefits of a distributed architecture can be retained. These advantages include: (i) faster scheduling; (ii) lower transmission latency; (iii) faster response to retransmissions: (iv) absence of a need for inter-cell or inter-site communication interfaces; (v) reduction in network complexity; and (vi) favorable architecture for hybrid automatic repeat requests, H-ARQ.